The present invention relates to a rubber composition for a sidewall and a pneumatic tire having a sidewall prepared using the composition.
So far a sidewall has been subject to degradation in its appearance due to ozone and ultraviolet rays in the air when used for a long period of time and cracks were occasionally generated on the sidewall. Further, migration of wax and an antioxidant compounded to its surface occurred, which often caused discoloration of tires and damaged the aesthetic aspect of tires.
As a procedure to overcome these problems, it has been known to compound wax which migrates to a tire surface and forms a film to physically prevent deterioration by ozone, etc. However, in such a case, there has been no specific measure as to how the composition of wax should be. Further, when inappropriate wax was compounded, there was a case where appearance of tires was damaged.
Further, as a means for preventing cracking attributable to ozone, it has been also known that a large amount of antioxidants is compounded. However, in this case, there has been a problem that the antioxidants are scattered and lost at an early stage of use and their effect cannot be maintained until the end of use.
JP2006-89612A discloses a rubber composition containing specified amounts of a rubber component, paraffin wax and a gelation agent and preferably used for a tread and a sidewall. However, a ratio of iso content to normal content of the wax to be used is not prescribed, and no consideration is made as to the point that a particularly superior effect is obtained by using a specific wax.
JP11-181150A discloses a rubber composition which is suitably used for a tread or a sidewall and comprises a rubber component and wax comprising normal paraffin in an amount of 75 to 85% by weight and iso paraffin. However, there are not described ratios of standard deviation/average number of carbon atoms of iso content and normal content of the wax being used, and no consideration is made as to the point that a particularly superior effect is obtained by using a specific wax.
Namely, there is no concept disclosed in JP2006-89612A and JP11-181150A with respect to spreading of iso content and normal content of wax being used and improvements are still required in an aesthetic aspect of tires.